The parathyroid is comprised of 4 small glands embedded in the posterior aspect of the thyroid gland. Its main function is the production and secretion of parathyroid hormone (PTH), a polypeptide hormone responsible for maintaining serum calcium homeostasis. The levels of PTH and serum calcium are inversely proportional. At low serum calcium levels, PTH, in conjunction with vitamin D, works at many sites in the body to mobilize calcium stores and increase calcium absorption and reabsorption. Both calcium and vitamin D provide negative feedback to the parathyroid glands; as levels of calcium and vitamin D increase, they bind receptors at the parathyroid glands and inhibit the production and release of PTH.